Method of selecting specific region of sample

ABSTRACT

This invention relates to a selection method of a specific region of a sample which is suitable for use in a nuclear magnetic resonance (NMR) imaging. Selection of a specific region suitable for obtaining multi-dimensional spatial information can be preferably made. To satisfy this requirement, the spin of the specific region of the sample is brought down in a direction at right angles to a static magnetic field, the regions other than the specific region are, in the mean time, brought down in a direction at right angles to the static magnetic field, and thereafter the static magnetic field is rendered non-uniform to that the spins of the regions other than the specific region become saturated.

DESCRIPTION

1. TECHNICAL FIELD

This invention relates to a method of selecting a specific region of asample, and more particularly to a method of selecting a specific regionof a sample which is suitable for NMR (nuclear magnetic resonance)imaging.

2. BACKGROUND ART

When a sample having atomic nuclei whose spin quantitization number isnot zero is placed in a static magnetic field, the atomic nuclei exhibita macro behaviour as an aggregate and can be regarded as a magnetizationvector having magnetization in parallel with the direction of the staticmagnetic field and angular momentum. The relation between a magneticmoment μ representing the magnitude of magnetization and the angularmomentum J is expressed as follows with γ representing a gyromagneticratio:

    μ=γJ                                              (1)

The equation of motion can be expressed as follows with H₀ representingthe intensity of the magnetic field: ##EQU1##

When eq. (1) is differentiated and put into eq. (2), there is obtainedthe following equation: ##EQU2## Eq. (3) can be expressed as follows bya coordinate system rotating at an angular velocity ω: ##EQU3## Thefrequency ω at which H_(e) =0 is referred to as a "resonance frequencyω₀ " (angular velocity expression) and is represented by the followingequation:

    ω.sub.0 =-y·H.sub.0                         ( 6)

If the direction of the static magnetic field H₀ is called "Z" anddirections orthogonal thereto are "X, Y" and a magnetic field H₁ isapplied from the X direction (this means a radio frequency magneticfield rotating at ω₀), the magnetization vector rotates on the Z - Yplane at an angular velocity expressed by γ·H₁. A magnetic field H₁which is applied until the magnetization vector is 90° relative to the Zaxis (falls on the X - Y plane) is referred to as "90° pulse", and "180°pulse" is defined similarly. The component of the X - Y plane of themagnetization vector is induced as a signal in a coil disposed on theX - Y plane. The description given above explains briefly the principleof nuclear magnetic resonance.

One of the conventional methods which selectively obtains a signal froma specific region of a sample on the basis of the principle describedabove applies a radio frequency magnetic field having frequencycomponents f₁ (=γh₁)˜f₂ (=γh₂) (where h₁ and h₂ represent the intensityof magnetic field in specific regions x₁ and x₂ of a sample in the xdirection, respectively) to the sample in the presence of a gradientmagnetic field in the x direction of the sample, and then nutates ortips by 90° the magnetization vector in that region in order toselectively excite the magnetization vectors of the specific region x₁˜x₂ of the sample.

This method is excellent to selectively obtain a signal in auni-dimensional direction. However, since the magnetization vector isbrought down by 90° for one selection, this method cannot be applied aplurality of times in order to make selections a plurality of times andto obtain spatial information on regions of a plurality of dimensions.In other words, in accordance with this method, selection for obtainingthe spatial information of two or more dimensions cannot be made.

Another conventional method picks up selectively only the frequencycomponents f₁ ˜f₂ corresponding to the specific region by bringing downby 90° the magnetization vectors for a wide range including the specificregion x₁ ˜x₂, then applying the gradient magnetic field and controllingthe frequency band of the resulting signal. This method is a so-called"frequency filter system" and includes a system by use of an analogfilter and a system using a digital filter in a narrow sense of theword. The analog filter has the disadvantage that the frequency bandcannot be changed arbitrarily. Furthermore, its expansion to the spatialselection of two or more dimensions is not possible. The digital filterincludes a digital filter in a narrow sense (that is, an ordinarydigital filter system utilizing fold-in) and a filter utilizing Fouriertransform.

The digital filter in a narrow sense has the advantage that thefrequency band can be changed arbitrarily, but its expansion to two ormore dimensions is not possible in the same way as the analog filter. Adata sampling period t_(int) for digitizing the signal must satisfy thefollowing condition from the Nyquist's theorem: ##EQU4## where f_(max)is a maximum frequency contained in the signal.

Accordingly, a sampling rate is determined by the signal band of theexcited signal quite irrelevantly to the frequency band that correspondsto the specific region.

The filter utilizing Fourier transform samples the signal, converts itto a digital signal, subjects altogether the sampled data to Fouriertransform and picks up only the data of the object frequency band (f₁˜f₂) from the resulting data. This Fourier transform method can separatethe spatial information of multiple dimensions by skilfully utilizingthe principle of nuclear magnetic resonance such as two-dimensionalFourier transform imaging method and three-dimensional Fourier transformimaging method, and moreover, arbitrary filtration is possible inprinciple. This method collects only the data relating to the specificdata and discards the rest from the data obtained by ordinaryprocessing. Therefore, this method is excellent in that any particularprocessing is not necessary.

In accordance with this Fourier transform method, too, the condition ofthe sampling rate expressed by the equation (7) must be satisfied. Onthe other hand, the data acquisition time T_(aq) must satisfy thefollowing condition in order to obtain resolution Δf: ##EQU5##Therefore, the number of data points N is given as follows from eq. (7)and (8): ##EQU6## If the frequency band corresponding to the objectspace is f⁰ _(max) resolution is Δf⁰, the necessary data point number Nis given as follows: ##EQU7## When the ratio of eq. (9) to eq. (10) isobtained, ##EQU8##

In other words, when the same resolution is maintained, a greater datapoint number becomes necessary with a higher selection ratio, and if thedata point number is kept the same, it means that resolution drops. Toavoid such a trade-off, it is by all means necessary to bring the objectregion into conformity with the signal generation region.

DISCLOSURE OF THE INVENTION

The present invention is directed to provide a method of selecting aspecific region of a sample which is suitable for obtainingmultidimensional spatial information.

In accordance with the present invention, there is provided a method ofselecting a specific region of a sample which includes the steps ofarranging a sample in a static magnetic field so that the spin in thesample is oriented parallel to the static magnetic field, bringing downthe spin of the sample in the specific region in a direction at rightangles to the static magnetic field and then orienting it parallel tothe static magnetic field so that the spin of the sample in regionsother than the specific region is brought down in a direction at rightangles to the static magnetic field, and making non-uniform the staticmagnetic field so that the spin of the sample in the regions other thanthe specific region becomes saturated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a nuclear magnetic resonance imagingapparatus for practising the method of the present invention inaccordance with one embodiment thereof;

FIG. 2 is a perspective view showing the state of arrangement of asample in a static magnetic field;

FIG. 3(A) is a schematic view showing the first state of magnetizationvector of the sample;

FIG. 3(B) is a schematic view showing the second state of themagnetization vector of the sample;

FIG. 3(C) is a schematic view showing the third state of themagnetization vector of the sample;

FIG. 3(D) is a schematic view showing the fourth state of themagnetization vector of the sample;

FIG. 3(E) is a schematic view showing the fifth state of the vector ofthe sample;

FIG. 3(F) is a schematic view showing the sixth state of the vector ofthe sample;

FIG. 4 is a time chart of a first embodiment of the method of thepresent invention;

FIG. 5 is a time chart of a second embodiment of the present method;

FIG. 6 is a time chart of a third embodiment of the present method;

FIG. 7 is a time chart of a fourth embodiment of the present method;

FIG. 8 is a time chart of a fifth embodiment of the present method;

FIG. 9 is a time chart of a sixth embodiment of the present method; and

FIG. 10 is a perspective view showing the state of arrangement of thesample and useful for explaining an application example of the presentmethod.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, a current is supplied from a magnet power source 12to a magnet 10 to generate a uniform static magnetic field H₀. A sample16 is disposed at the center of the magnet 10. A radiation coil 18 isconnected to a power source 22 through a radio frequency (RF) poweramplifier 20. The power amplifier 20 receives a pulse generated by afrequency synthesizer 24 through a radio frequency (RF) transmitter 26and an amplitude modulator 28 and applies a radio frequency (RF) voltageto the radiation coil 18. A gradient magnetic field coil device 14 isconnected to an X gradient magnetic field driver 30, a Y gradientmagnetic field driver 32 and a Z gradient magnetic field driver 34 andgenerates gradient magnetic fields in the X, Y and Z direction by acomputer 38 which is connected to each of the gradient magnetic fielddrivers through an interface 36.

A reception coil 40 detects a nuclear magnetic resonance (NMR) signalfrom the sample 16 and the detected NMR signal is applied to a phasedetector 46 through a pre-amplifier 42 and a radio frequency (RF)receiver 44. The phase detector 46 is connected to the RF transmitter26, detects the phase of the resonance signal using the frequency of thevoltage applied to the radiation coil 18, that is, the frequency of theRF magnetic field, as the reference signal, and applies it to an audioamplifier and a filter 48. The audio amplifier and the filter 48 selectpredetermined NMR signals and display them on a display 52 through andA/D convertor 50 and the interface 36. Reference numeral 54 representsan operation panel of the computer 38.

It will be assumed that the sample 1 as a subject is arranged as shownin FIG. 2 and the Z axis represents the direction of the static magneticfield H₀. The X and Y axis represent in this case two axes that areperpendicular to the Z axis and orthogonally cross each other. Thoughselective saturation in the X direction will be described in thisembodiment for convenience' sake, the same principle can be applied tothe other axes.

In FIG. 2, a hatched portion (A˜B) is the specific region, that is, theobject region.

FIG. 3 is a view when viewed in the direction of arrow 56 in FIG. 2.When the gradient magnetic field acts, the magnetization vectors of thesample 1 face in the Z direction throughout the entire regions.Therefore, a gradient magnetix field G_(x) having different magneticintensity in accordance with positions in the X direction is applied tothe static magnetic field. Next, 90° pulse selective radiation iseffected for the object region A˜B. In other words, an RF magnetic fieldhaving concentratedly a component between a resonance frequencycorresponding to the magnetic field intensity of the point A and aresonance frequency corresponding to the magnetic field intensity of thepoint B is applied to the sample, in order to selectively excite themagnetization vectors of the portion of the region A˜B of the sample 1and to bring down the spins or vectors on the X - Y plane, as shown inFIG. 3(B).

When selective radiation is effected and the magnetization vectors ofthe region A˜B are brought down on the X - Y plane, the magnetizationvectors on the X - Y plane cause phase disturbance. This is because thestatic magnetic field H₀ is not completely uniform and the gradientmagnetic field is applied. Therefore, the phases must be made uniform.To make them uniform, an echo is generated. This echo is generated byapplying once again the gradient magnetic field G_(x) and by applying a180° pulse. At this time, the direction of the magnetization vectors isinverted such as shown in FIG. 3(C).

Thereafter, a non-selective 90° pulse is applied. That is to say, the90° pulses are applied to the entire regions of the sample 1 as shown inFIG. 3(D) to direct the magnetization vectors between the region (A˜B)in the direction of the Z axis and to bring down the magnetizationvectors of the rest of portions onto the X - Y plane. Next, G_(x) isapplied in order to impart non-uniformity (homo spoil pulse) of themagnetic field. G_(Y) or G_(Z) may be applied in place of G_(x). As aresult, the components of the magnetization vectors on the X - Y planedisappear due to the phase disturbance, and the nuclear spins in theregions other than the region A˜B can be saturated (FIG. 3(E)).

As shown in FIG. 3(F), therefore, among the magnetization vectors of thesample 1, only the magnetization vectors of the portion A˜B can be left.For this reason, the NMR signal is not generated from the regions otherthan the object region and the necessary data number can be decreased.

The application timing of the gradient magnetic field G_(x), the RFmagnetic field and the generating timing of the echo is shown in FIG. 4,where S₁, S₂ and S₃ are as follows, respectively; ##EQU9## Theapplication of G_(x) is stopped at a timing t₅ which satisfies thefollowing relation:

    S.sub.1 +S.sub.2 =S.sub.3                                  (13)

The 180° pulse is generated at t₄ and the echo is applied at t₆ at whicht₁ ˜t₄ is equal to t₄ ˜t₆. The 90° pulse is applied to t₆. (B)˜(E)correspond to those of FIG. 3, respectively.

Next, the magnitude of the magnetization vectors that attenuate duringthe operation described above will be discussed. Since the 180° pulseecho method is employed, the attenuation quantity in the period t₁ ˜t₆depends on T₂ which is a spin-spin relaxation time. If T₂ is at least 60msec, the attenuation quantity is 1-l^(-6/60) =0.10 and maximum 10% ift₁ ˜t₆ is below 6 msec. This renders no problem in consideration of thefact that t₁ ˜t₆ can be reduced further and T₂ is generally greater thanthe value described above.

When the operation described above is effected for G_(Y) or G_(Z), orboth of G_(Y) and G_(Z), two- or three-dimensional object regions can beselected while the magnetization vectors of the other regions can besaturated.

As the echo generation means shown in this embodiment, t₂ may be inagreement with t₃ as shown in FIG. 5 or G_(x) may be appliedcontinuously during t₁ ˜t₄. In the embodiment of the invention shown inFIG. 6, the following condition must be satisfied: ##EQU10##

As the echo generation means shown in the embodiment, G_(x) may beapplied as shown in FIG. 7 with the proviso that S₁ +S₃ =S₂ must besatisfied.

The polarity of G_(x) may be switched as shown in FIG. 8 as the echogeneration means shown in the drawing with the proviso that S₁ =S₂.

The 180° pulse shown in the embodiment may be the selective radiationpulse or the non-selective pulse.

The non-uniform magnetic field to be applied originally after completionof the application of the 90° pulse may be applied simultaneously withthe 90° pulse provided that the application time of the 90° pulse to beapplied at t₆ in the embodiment is extremely short and thenon-uniformity of the magnetic field is such that the phase disturbancewithin the application time can be neglected. In this instance, thegradient magnetic field applied in order to generate the echo may beapplied by the same gradient magnetic field as the non-uniform magneticfield described above. In other words, the gradient magnetic field maybe applied continuously before and after the 90° pulse shown in FIG. 9.

It is also possible to employ the arrangement wherein FIGS. 3(B) and (C)are omitted, the non-selective 90° pulse is applied in the presence ofG_(x) to bring down by 90° the spins of the sample as a whole and thenthe selective 90° pulse having a different polarity is applied in thepresence of G_(x) so as to return the spins of the region A˜B to thedirection of the static magnetic field as shown in FIG. 3(D).

In the embodiment described above, the RF magnetic field is appliedalong the Y axis. However, it may be applied in an arbitrary directionso long as the axis is on the X - Y plane.

FIG. 10 is an explanatory view for explaining the application example ofthe NMR signal detecting method in accordance with the presentinvention.

The selective saturation method described already is effected for eachof the X and Y axes to select two-dimensionally the sample region asshown in FIG. 10. The sample region is sliced in the direction of the Zaxis. Thereafter, an ordinary spin warp method is practised for imaging.The spin warp method is described in detail in Edelsteim W. A.,Hutchison J. M. S., Johnson G. and Redpath T., "Spinwarp NMR imaging andapplications to human whole-body imaging", Phys. Med. Bio. (1980; 25:751-756).

The sampling data point number N_(x) is given as follows with Xrepresenting the frequency encoding direction, Y representing the phaseencoding direction and f_(max),x representing the maximum frequency inthe direction of the X axies expressed by eq. (9): ##EQU11## Since themaximum value of X is small by the selective saturation method,f_(max),x becomes small, and N_(x) can be eliminated by making Δfconstant. On the other hand, the data point number can be decreased inthe phase encoding direction, too. Namely, the number of times of phaseencoding can be decreased and the number of times of scanning can bereduced. The measurement time T can be expressed as follows:

    T=t.sub.SCAN ×n

where t_(SCAN) is a scanning time and n is the number of times ofscanning.

If the number of times of scanning can be decreased, the measurementtime can be reduced. This means that the restriction time of a subjectcan be shortened and his pain can be mitigated. Eq. (9) can be modifiedas follows: ##EQU12## If f_(max) is made small by the selectivesaturation method, resolution Δf can be made small if N is keptconstant. In other words, the accuracy of resolution can be improved.

Next, the second application example will be described.

The selective saturation method of the present invention is carried outin the direction of the X axis for a sample that moves in the Xdirection with the passage of time. After the passage of a suitableperiod, the magnetization vectors are excited at an arbitrary point inthe X direction and the signal is detected.

The moving speed of the sample can be determined from the time passedafter practising the selective saturation method and the signaldetection position in the X direction.

What is claimed is:
 1. A method of selecting a specific region of asample including the steps of: arranging the sample in a static magneticfield to thereby orient the spins of the sample in a direction parallelto the static magnetic field; bringing down selectively the spins of thespecific region of the sample in a direction perpendicular to the staticmagnetic field; inverting the brought down spins and orienting theinverted spins in the direction parallel to the static magnetic fieldwhile bringing down the spins of regions of the sample other than thespecific region in the direction perpendicular to the static magneticfield; and making non-uniform the static magnetic field to therebysaturate the spins of the regions other than the specific region.
 2. Amethod of selecting a specific region of a sample including the stepsof: arranging the sample in a static magnetic field to thereby orientthe spins of the sample in a direction parallel to the static magneticfield; bringing down selectively the spins of the specific region of thesample in a direction perpendicular to the static magnetic field;inverting the brought down spins; brining down by 90° the inverted spinsand the spins of regions of the sample other than the specific region tothereby orient the inverted spins in the direction parallel to thestatic magnetic field and the spins of regions of the sample other thanthe specific region in the direction perpendicular to the staticmagnetic field; and making non-uniform the static magnetic field tothereby saturate the spins of the regions of the sample other than thespecific region.
 3. A method of selecting a specific region of a sampleincluding the steps of: arranging the sample in a static magnetic fieldto thereby orient the spins of the sample in a direction parallel to thestatic magnetic field; applying a first radio frequency pulse having aselected frequency component to the sample to thereby bring down thespins of the specific region of the sample in a direction perpendicularto the static magnetic field; applying a second radio frequency pulse tothe sample to thereby invert the brought down spins; applying a thirdradio frequency pulse to the sample to thereby orient the inverted spinsin the direction parallel to the magnetic field and the spins of regionsof the sample other than the specific region in the directionperpendicular to the static magnetic field; and making non-uniform thestatic magnetic field to thereby saturate the spins of the regions ofthe sample other than the specific region.
 4. A method of selecting aspecific region of a sample including the steps of: arranging the samplein a static magnetic field to thereby orient the spins of the sample ina direction parallel to the static magnetic field; applying a firstradio frequency pulse having a selected frequency component to thesample to thereby bring down the spins of the specific region of thesample in a direction perpendicular to the static magnetic field;applying a second radio frequency pulse to the sample to thereby invertthe spins of the sample; applying a third radio frequency pulse to thesample to thereby bring down the inverted spins by 90° so as to orientthe spins of the specific sample in the direction parallel to the staticmagnetic field and the spins of regions of the sample other than thespecific region in the direction perpendicular to the static magneticfield; and applying a magnetic field gradient to the static magneticfield to thereby make non-uniform the static magnetic field so as tosaturate the spins of the regions of the sample other than the specificregion.